Craziest Internet Explorer Bug Ever?

Craziest Internet Explorer Bug Ever?

As web developers, we’re used to working around the bugs in Microsoft Internet Explorer. The older versions are worst, and I’m certainly glad to not have to write code that works in Internet Explorer 6 (or, increasingly, Internet Explorer 7) any more: even Microsoft are glad to see Internet Explorer 6 dying out, but even IE8 is pretty ropey too. And despite what Microsoft claim, I’m afraid IE9 isn’t really a “modern” browser either (although it is a huge step forwards over its predecessors).

But imagine my surprise when I this week found what I suspect might be a previously undiscovered bug in Internet Explorer 8 and below. Surely they’ve all been found (and some of them even fixed), but now? But no. It takes a very specific set of circumstances for the bug to manifest itself, but it’s not completely unbelievable – I ran into it by accident while refactoring parts of Three Rings.

A completely useless Internet Explorer error message.

A completely useless Internet Explorer error message. Thanks, IE.

Here’s the crux of it: if you’re -

  • Using Internet Explorer 8 or lower, and
  • You’re on a HTTPS (secure) website, and
  • You’re downloding one of a specific set of file types: Bitmap files, for example, are a problem, but JPEG files aren’t (Content-Type: image/bmp), and
  • The web server indicates that the file you’re downloading should be treated as something to be “saved”, rather than something to be viewed in your browser (Content-Disposition: attachment), and
  • The web server passes a particular header to ask that Internet Explorer does not cache a copy of the file (Cache-Control: no-cache),

Then you’ll see a dialog box like the one shown above. Switching any of the prerequistes in that list out makes the problem go away: even switching the header from a strict “no-cache” to a more-permissive “private” makes all the difference.

I’ve set up a test environment where you can see this for yourself: HTTP version; HTTPS version. The source code of my experiment (PHP) is also available. Of course, if you try it in a functional, normal web browser, it’ll all work fine. But if you’ve got access to a copy of Internet Explorer 8 on some old Windows XP box somewhere (IE8 is the last version of the browser made available for XP), then try it in that and see for yourself what a strange error you get.

Dan Q is a software engineer, a director of a voluntary organisation, a trainee counsellor, a keen geocacher, and an amateur magician. He lives with his partner and her husband in a polyamorous triad, and occasionally finds time to blog.